1. Field of the Invention
This invention relates generally to a brushless motor and, more particularly, to a brushless motor in which a rotor position sensor is not required.
2. Description of the Background
A conventional brushless motor typically detects the angular position of the rotor by using a position sensor, such as a Hall-effect element. Upon determining the rotational position of the rotor a switching pulse is produced that switches the phases of the stator coil energization based upon the detected signal. Use of the Hall-effect elements, however, increases the cost of the motor and also increases the complexity of the wiring and the number of steps employed in assembling the motor.
This applicant has previously proposed a brushless motor that does not require a position sensor, such as the known Hall-effect element, and this proposed brushless and sensorless motor is described in Japanese Patent Application No. 61-125413. In this proposed brushless motor, the voltage induced in the stator coil by the rotor magnet is detected and utilized to form pulses that indicate specific angular positions of the rotor, and this pulsed signal is delayed by a fixed value to form the energization timing pulses at the respective switching positions.
In order to produce these stator coil energization timing pulses in the above-identified previously proposed sensorless brushless motor, a fixed time delay device is employed, such as a monostable multi-vibrator. This proposed system has the drawback in that it is not applicable to motors in which the speed is controlled over a relatively wide range. For example, the brushless motor utilized in a constant linear velocity (CLV) kind of video disk player cannot easily be implemented with the previously proposed system. In the proposed sensorless brushless motor in order to start the motor, special energization pulses are generated at fixed intervals and with fixed durations regardless of the initial angular position of the rotor. Therefore, the energization angle of the stator coils is not synchronized with the rotational angle of the rotor resulting in poor starting characteristics.
Furthermore, because the angular position of the rotor is detected based on the induced voltages in a stator excitation coil, substantial noise, in the form of spikes, is generated at each energization switching point in such coil. The noise spikes, as well as other noises in the detection system, are then mixed in the detection signal that is employed to determine rotor angular position. Therefore, because of this noise the rotor energization switching angles, the duration of the energization pulses, as well as the polarities of the energization pulses cannot be determined in a stable fashion.